The size of the universe refers to the relative size of the universe and entities contained within it. The universe is vast; it has been expanding for almost 13.8 billion years at a considerable fraction of the speed of light, so it is roughly 13 billion light years across. This means that the universe is ~1.3 x 1026 m across. For a reference of size, humans are usually under 2 m tall, atoms are approximately 10-10 m across, and nuclei are approximately 10-15 m across. There is a difference of a factor of 1041 between the universe and a nucleus, which can be written out as 100 000 000 000 000 000 000 000 000 000 000 000 000 000, a number so large it is hard to wrap one's head around.
Here are some useful on-line tools that compare the sizes of various objects in the universe:
Here is a video from IMAX cosmic voyage which demonstrates size comparisons between various entities (music from "The Matrix" movie-trilogy and "Awake" by Jake Kaufman):
Please contact us with other tools showing the size of the universe in interesting ways.